1. Field of the Invention
One or more embodiments of the present invention relate to a phosphorus containing benzoxazine-based monomer, a polymer thereof, an electrode for a fuel cell including the polymer, an electrolyte membrane for a fuel cell including the polymer, and a fuel cell using the electrode
2. Description of the Related Art
Fuel cells that use a polymer electrolyte membrane as an electrolyte operate at a relatively low temperature and can also be small in size, and thus they are expected to be used as an electric power source in electric vehicles or distributed generation systems for homes. As a polymer electrolyte membrane used in polymer electrolyte fuel cells, perfluorocarbonsulfonic acid-based polymer membranes represented by NAFION® (DuPont Company) are used.
However, such polymer electrolyte membranes need water to retain proton conductivity, and thus the polymer electrolyte membranes need humidifying. In addition, to enhance cell system efficiencies, it is necessary to operate polymer electrolyte membranes at a high temperature of at least 100° C. However, the moisture in polymer electrolyte membranes evaporates at this temperature, and polymer electrolyte membranes can not function as a solid electrolyte.
To address those problems in the art, non-humidified electrolyte membranes which can operate at a high temperature of at least 100° C. under nonhumidified conditions have been developed. For example, U.S. Pat. No. 5,525,436 discloses polybenzimidazole doped with phosphoric acid, and the like as a material constituting non-humidified electrolyte membranes.
In addition, in fuel cells that operate at a low temperature, such as fuel cells using a perfluorocarbonsulfonic acid-based polymer membrane, to prevent gas diffusion in electrodes because of water (formation water) that is produced as electricity is generated in an electrode, particularly a cathode, electrodes using polytetrafluoroethylene (PTFE) as a waterproof agent having hydrophobic properties have been widely used (for example, Japanese Patent Laid-Open Publication No. hei 05-283082).
In addition, phosphoric acid type fuel cells operating at a high temperature of 150 to 200° C. use liquid phosphoric acid as an electrolyte. However, electrodes then may include a large amount of liquid phosphoric acid and that interferes with gas diffusion. Therefore, an electrode catalyst layer that is formed by adding polytetrafluoroethylene (PTFE) as a waterproof agent to an electrode catalyst, and that can prevent fine pores in electrodes from being clogged by phosphoric acid, has been used.
In addition, in fuel cells using a polybenzimidazole (PBI) electrolyte membrane that retains phosphoric acid as a nonhumidified electrolyte at a high temperature, in order to reduce contact between the electrodes and the electrolyte membrane, methods of impregnating electrodes with liquid phosphoric acid and of increasing the loading amount of metal catalysts have been tried. However, such fuel cells do not exhibit improved properties, and thus there is a need for further improvement.
In addition, in the case of supplying air to a cathode in a solid polymer electrolyte doped with phosphoric acid, the fuel cell requires an aging time of about 1 week even if the composition of the cathode is optimized. By supplying oxygen to the cathode instead of air, performance of the cathode can be improved and the aging time can also be reduced. However, supplying of oxygen to the cathode is a commercial obstacle to realizing widespread use of the cathode. In addition, a polymer electrolyte membrane formed from PBI does not have satisfactory mechanical properties and chemical stability at a high temperature and does not have the capability of retaining phosphoric acid.